1. Field of the Invention
This invention relates to digital communication systems, and in particular to an apparatus and method for measuring the average or center frequency of a frequency shift keyed signal which alternates regularly between two unknown frequencies with a known frequency shift and at a known key rate.
2. Description of the Prior Art
Frequency shift keying (FSK) is one way of conveying binary information in which the carrier switches or alternates between two predetermined frequencies. This may be accomplished for example by either frequency modulating one sine wave oscillator or by switching between two oscillators. In either case the alternating frequencies can be considered as deviating equally above and below an average, center or carrier frequency with a total frequency deviation referred to herein as the frequency shift or shift frequency.
Frequency shift keyed communication signals may be used in communication systems to alert the start or end of a transmission or some special event during a transmission, to synchronize remote equipment, or to transmit data. Under these circumstances detection of the onset of such an FSK signal, either by a communications receiver or by an intercept receiver, is a necessary system function.
In some situations, the transmission carrier frequency of the FSK signal may not be known at the receiver due to doppler shift, for example, or due to drift or wandering of the carrier frequency at the transmitter or due to lack of transmitter and receiver coordination. Unknown carrier frequency makes the onset detection of these signals more difficult. Furthermore, it may be necessary to determine the transmission carrier frequency for other system functions such as synchronization or demodulation, or in the case of a non-cooperative transmitter, for intercepting subsequent transmissions or for directing jamming signals.
Effective methods and apparatus for detecting the onset of such FSK signals are described in U.S. patent application, Ser. No. 340,741 entitled "Apparatus and Method for Detecting the Onset of a Frequency Shift Keyed Signal", filed by C. L. Bennett and R. L. Price on Jan. 20, 1982, and in U.S. patent application, Ser. No. 352,677 entitled "Improved Apparatus and Method for Detecting the Onset of a Frequency Shift Keyed Signal", filed by Richard R. Kurth on Feb. 26, 1982. Both applications are assigned to Sperry Corporation. One advantage of these inventions, as disclosed in the applications therefore, is that the onset detection scheme does not require knowledge of the FSK carrier frequency or of the time instance at which the frequency shifts occur. Only the amount of the frequency shift and its shift rate or key rate are required for the detection scheme. Those inventions also discriminate against other modulated or unmodulated tonal signals with properties different from the alternating FSK waveform, such as a pair of continuous wave (CW) signals separated in frequency by an amount equal to the FSK shift frequency. As a consequence of their operating principals, these detectors do not provide an estimate of average, center, or carrier frequency of the detected FSK signal. Thus it would be desirable to provide a frequency measuring system that is compatible with the onset detection schemes described in the referenced patent applications. To be fully compatible therewith the frequency measuring system should be capable of measuring the average frequency of the alternating FSK waveform when it is present as one signal among other interfering signals and noise, with a priori knowledge of only the FSK shift frequency and shift rate or key rate.
Several approaches are possible for measuring the average frequency of an alternating FSK signal whose onset has been detected by onset detectors such as described in the referenced patent applications, or by other means. First, a simple frequency discriminator in parallel with the detector could be used to measure frequency. However, the presence of any other tonal signals in the frequency band of observation could produce erroneous frequency estimates with this approach.
A second approach to alternating FSK signal frequency measurement is spectral analysis, accomplished, for example, by passing the input signal through a bank of narrow bandpass filters covering the input frequency band. The presence of signal energy in a particular filter output in excess of that in neighboring filter outputs signifies a tonal signal and thereby localizes it in frequency to that particular filter's pass band. If the spectral analysis frequency resolution (analysis filter bandwidth) is insufficient to resolve the individual FSK tone frequencies, it is difficult to determine reliably which frequency among the multiple frequencies revealed by the spectral analysis, corresponds to the detected alternating FSK waveform. If the analysis bandwidth is narrow enough to resolve the individual FSK tone, the presence of twin spectral components separated by the FSK shift frequency potentially identifies the frequency location of the alternating FSK signal. However, spectral analysis with this degree of resolution involves substantial signal processing and still does not permit easy discrimination against two unmodulated tones with a frequency difference equal to the FSK shift frequency.
A third approach is to filter the input signal with a bank of frequency contiguous bandpass filters of bandwidth not less than the FSK shift frequency, and to process the output of each filter with an alternating FSK detector of the type described in the referenced applications, for example. This approach provides simultaneous detection of the alternating FSK signal and an estimate of its frequency by virtue of the narrow frequency band in which detection occurs. The individual alternating FSK detectors provide the necessary discrimination against other tonal signals with different characteristics. However, this approach is complex, requiring parallel channels each with a filter and detector. A variant of this approach with fewer parallel channels involves several sequential stages of filter banks and detectors with increasing frequency resolution. However, this approach has the disadvantage of introducing a delay into the frequency measurement process and requires more control circuitry.